Exhaust gas emitted from automobiles equipped with a diesel engine or the like contains substances such as nitrogen oxides (NOx). In recent years, various measures haven been taken to purify such exhaust gas in order to protect as well as to prevent contamination of the environment. One measure used to purify exhaust gas is an exhaust gas purification apparatus.
Such exhaust gas purification apparatus is mounted in an automobile and decomposes nitrogen oxides (NOx) to render them harmless by means of an NOx selective catalytic reduction (SCR) system. The NOx selective catalytic reduction (SCR) system employs a urea aqueous solution as a reducing agent. The urea aqueous solution is contained in a tank mounted in the automobile. In order to effectively decompose nitrogen oxides (NOx), the concentration of the urea aqueous solution (the concentration of urea in the urea aqueous solution) must be maintained within a specified range.
Even when a urea aqueous solution of appropriate concentration is charged in the tank, the concentration of the urea aqueous solution may fall outside the specified range, due to changes occurring with the passage of time or the like. Also, a worker may erroneously mix light oil or water in the tank. In order to cope with these problems, a device for determining the concentration of urea in the urea aqueous solution contained in the tank has been proposed as a sensor for detecting whether or not the urea aqueous solution is within a suitable concentration range (Patent Document 1).
The urea-concentration-determining device disclosed in Patent Document 1 includes a concentration-determining sensor section and a support section. The concentration-determining sensor section has a concentration-detecting portion, which includes a heating element and a temperature-sensing element, and a liquid-temperature-detecting portion for measuring the temperature of the urea aqueous solution.
The support section is located above the concentration-determining sensor section and has an attachment portion attaching to an opening section of the urea-aqueous-solution tank, a circuit board located above the attachment portion, and a tubular member located below the attachment portion and adapted to hold the concentration-determining sensor section. The circuit board of the support section has a concentration detection circuit and is covered with a cover member. The concentration-detecting portion and liquid-temperature-detecting portion of the concentration-determining sensor section are electrically connected to the circuit board through respective lead wires which extend through the tubular member.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2005-84026
In the case where a terminal member is fixedly attached to a sensor element, and an electrical communication member, such as a lead wire or a cable, is connected to the terminal member by soldering or the like, a need may arise for a configuration where at least a portion of the terminal member and at least a portion of the sensor element are disposed within a holder tube (tubular member), and where the holder tube directly or indirectly holds the sensor element so as to prevent entry of liquid into the holder tube.
However, an assembled state of the above configuration may involve a risk of contact and a resultant short circuit between the holder tube and the terminal member. Also, there may be a risk of contact and a resultant short circuit between the holder tube and the sensor element or a risk of cracking or the like in the sensor element. In the case where a plurality of terminal members are present, there may be a risk of contact and a resultant short circuit between the terminal members. Furthermore, vibration or impact during actual use vibrates the sensor element, the terminal member, the lead wire, and the like and consequently generates stress in a connection between the lead wire and the terminal member or a connection between the terminal member and the sensor element. Such vibration or impact potentially induces cracking or a like problem in these connections or portions of the terminal member and the sensor element in the vicinity of the connections.